Wavelength division multiplexing is the transmission of several different signals via a single optical fibre, by sending each signal (“channel”) at a different optical frequency or wavelength. A multiplexer is used to combine the different channels together for transmission, and a demultiplexer is used to separate the channels following the transmission.
WDM optical transmission systems are typically composed of a number of spans of optical fibre, and include a variety of network elements such as terminals, line amplifiers, and add/drop nodes.
It is known that it is desirable to control the power levels of each of the channels in a WDM system. Optical signals experience wavelength dependent effects as they are transmitted between network nodes. Examples of wavelength dependent effects include fibre attenuation, Raman scattering within the optical fibre, optical amplifier wavelength dependent gain, chromatic dispersion slope, and variation in the uniformity of optical filters.
Due to the wavelength dependent effects, each channel will experience different impairments during transmission, potentially leading to each channel having a different bit-error rate (BER). The BER expresses the percentage of bits received in error, for example by an optical receiver, as compared to the total number of bits received. Another metric for measuring the quality of an optical channel is the Optical Signal to Noise Ratio (OSNR).
To offset the different propagation impairments experienced by the different channels, it is known to control the power of each channel. For example, it is known to use channel power pre-emphasis to control the transmission power of each channel, for offsetting the propagation impairments likely to be experienced by each channel as all of the channels are transmitted from a transmitter to a receiver over a single, linear link.
A variety of channel power pre-emphasis techniques are known, with different techniques often using different quality metrics to determine the channel transmission powers. As for instance described in U.S. patent application Ser. No. 11/572,549, power pre-emphasis techniques can be based upon equalisation of the OSNR, or the BER.